In a mixing bowl, combine the instant pistachio pudding mix with 2 cups of cold milk. Whisk until well blended, and let it sit for about 5 minutes to thicken.

Once the pudding has thickened, gently fold in the crushed pineapple until evenly distributed.

Fold in the mini marshmallows.

Add the chopped walnuts or pecans to the mixture and stir until well combined.

Gently fold in the whipped topping until everything is well mixed.

Transfer the mixture to your serving dish and cover it with plastic wrap. Chill in the refrigerator for at least 2 hours.

Once chilled, serve the Watergate dessert in bowls or on plates.